Much of the oil in most reservoirs remains in place after all present-day methods of production have been applied to recovery. The U.S. Department of Energy estimates currently that 70% to 80% of the oil that has been discovered in the United States remains underground. This is true in much of the world, and in the case of viscous, or “heavy”, oil deposits the amount left in the ground after all known methods of enhanced production have been applied exceeds 90%. With oil shale there is very little hydrocarbon production because of the small amount of hydrocarbon associated or bonded with a large amount of inorganic material (rock or shale).
Many enhanced recovery means have been developed and applied to oil reservoirs, including a method called “fire flooding”, whereby combustion is initiated in the oil reservoir and is sustained by injecting air. The heat from combustion lowers the viscosity of the oil deposit, the combustion product gases generate pressure, and the result is the “driving” of oil from the injection well in which the combustion is initiated toward a production well a distance away.
Fire flooding is operated so as to produce the maximum amount of oil and to minimize the amount of oil converted to combustion gases. The carbon dioxide and attendant nitrogen from the injected air sustaining combustion are major diluents in the gas produced. The objective of “fire flooding” is to produce the maximum amount of oil recoverable by this method and the gas made is not a useful product.
In the case of oil shale, no commercially successful method of production has been as yet developed for the vast U.S. and other deposits of oil shale.